Eclipse/BSD's CPU scheduler uses the MTR-LS (Move-To-Rear List Scheduling) algorithm [6]. When a process blocks (e.g., waiting for I/O), MTR-LS keeps the unused portion of the process's quota in the same position in the scheduling list, unlike the Weighted Round Robin (WRR) algorithm, which removes the process from the runnable list and, when the process becomes runnable again, places it back at the tail of the list. Consequently, MTR-LS may delay I/O-bound processes much less than does WRR. MTR-LS may also provide greater throughput than does WRR, whose scheduling delays may prevent I/O-bound processes from from fully utilizing their CPU reservations.
MTR-LS was specifically designed for CPU scheduling, where the time necessary to process a request cannot be predicted. To the best of our knowledge, MTR-LS is the only algorithm that provides the optimal cumulative service guarantee [6] when the durations of service requests are unknown a priori. However, MTR-LS assumes that requests can be preempted either at any instant or at fixed intervals. This is true of CPU scheduling, but usually is not true of disk or network scheduling, where requests cannot be preempted after they start and may take varying time to complete. Therefore, Eclipse/BSD uses other algorithms for I/O scheduling.